This invention relates to the preparation of moisture impervious red phosphors. More particularly, this invention relates to coating red phosphor particles to impart moisture resistance.
Red phosphors known until recently are rare, and they do not have long persistence. U.S. Pat. No. 5,650,094 to Royce et al discloses rare earth activated divalent titanate phosphors, such as calcium titanate or calcium-zinc-magnesium titanate, but the emission is visible for only a few minutes. Lindmayer, U.S. Pat. No. 5,043,096 reported a strontium sulfide based phosphor activated with three rare earths in the form of their oxides, and fluxed with a halide such as LiF. However, the fired phosphor was highly sintered and had to be ground to obtain a useful powdered material. But grinding degrades the emission and the ground phosphor has to be heated or annealed to repair crystal defect damage. However, the emission performance is never fully restored. These phosphors are described as useful for paint formulations.
Alkaline earth sulfides, such as calcium sulfide, strontium sulfide, magnesium sulfide and barium sulfide, have long persistence, but they are degraded after exposure to moisture. Calcium sulfide is the least reactive, and barium sulfide is most reactive with water. When exposed to a high humidity atmosphere, or to a liquid that contains water, the sulfides decompose. These materials are useful for the preparation of luminescent materials such as phosphors. However, their utility as phosphors is limited because of their moisture sensitivity. They must be encapsulated, as with a polymer or plastic, or mixed with a water impervious material such as a paint to improve their moisture resistance.
Long persistent red phosphors such as calcium sulfide or strontium sulfide are activated with the divalent rare earth element europium and trivalent rare earth elements such as praseodymium (Pr), neodymium (Nd) samarium (Sm), cerium (Ce), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho) erbium (Er) thulium (Tm) and lutecium (Lu). Similar phosphors, but without the long persistence property, are useful in electroluminescent devices and cathode ray tubes. Long persistence in display devices however causes smearing of moving objects.
These phosphors can be made, as described in U.S. Pat. No. 4,725,344 to Yocom et al, by heating an alkaline earth metal halide and the activators to the melt temperature of the halide, and contacting the molten halide with hydrogen sulfide. The resulting alkaline earth metal sulfide can be formed into a dense, highly crystalline layer at comparatively low temperatures. Thus this method permits formation of thin phosphor films on glass substrates.
Powdered halide phosphors can be mixed with powdered alumina, which retards sintering of the phosphor to form a powdered phosphor. This powder can be added to a matrix material, such as powdered or molten plastic, fibers and the like, that encapsulate or encompass the phosphor, protecting it against the effects of moisture.
Activated strontium sulfide phosphors can also be made by firing at about 1200xc2x0 C. in the presence of vaporized sulfur passed over a carbon source. This process forms an atmosphere of carbon disulfide. A fine particle size phosphor is obtained.
By long persistence phosphors it is meant that the spectral emission of the phosphors after excitation with light, emit red or orange-red colored light for several hours after the light source is removed. Such phosphors have many uses and are highly desirable for safety equipment for example.
Thus a search for an effective moisture resistant barrier layer for a red-emitting phosphor has continued. Most organic materials, such as polymers and plastics, are water permeable, and they are unable alone to form a long lasting moisture impervious phosphor coating. Fluoride coatings applied from aqueous media have also been tried, but they can degrade the luminescence properties of the phosphors. Thus a method of coating alkaline earth based phosphor particles which provide a moisture barrier but does not degrade the luminescent properties of the phosphor particles would be highly desirable.
Alkaline earth phosphor particles are dispersed in a polar anhydrous solvent which contains a low concentration of a reactive fluoride, to apply an adherent, transparent fluoride coating over the phosphor particles. The coating does not degrade the luminescent properties of the phosphor. After exposure, the phosphor particles are washed with a pure anhydrous solvent and dried. The resultant phosphor particles have excellent long term stability.